Many of the ADCs that have been used in instrumentation or communications systems, including artificial satellites and wireless base stations, in recent years have been differential input ADCs. This is because the signal amplitude of each line of differential input is half that of single-ended input and therefore, distortion is rare and the even-order distortion and the in-phase component noise that are generated by circuits in front of the ADC are canceled by the differential input of the ADC, making it possible to realize the properties of broad band, low noise, and low distortion. However, when the input signals are single-ended signals, it is necessary to set up in front of the ADC a signal converter that converts single-ended signals to differential signals.
A single end to differential signal converter having a broad-band pulse transformer 10 with a midpoint input terminal on the secondary side such as that shown in FIG. 1 is generally used for this type of converter. ADC 11 has a single power source 102 and therefore, gives in-phase voltage signals 101 that are half the power source voltage to the midpoint on the secondary side of broad-band pulse transformer 10. Broad-band pulse transformer 10 is generally a passive component capable of transmitting high frequencies of 500 MHz or higher and theoretically, it has the excellent advantages of not generating noise, as well as not generating distortion as long as the core remains unsaturated. Thus, the high-frequency properties of ADC 11 with low-noise and low-distortion performance can be realized indefinitely. However, because the transformer does not allow low-frequency signals to pass, it cannot be used for, for instance, the front end circuit of measuring apparatuses that measure direct current and low frequency signals, such as noise analysis that includes low frequency (FFT).
A circuit that makes inverting and non-inverting differential signals while providing the in-phase voltage signals 101 necessary for ADC 11 input using ordinary operational amplifiers 12 through 14 as shown in FIG. 2(a), or a circuit that uses an ordinary power source having in-phase voltage signal input and ordinary differential amplifier 15 as shown in FIG. 2(b), is generally used when processing signals that include direct current signals and low-frequency signals. However, these converters use dynamic components and therefore, noise is generated in an amount that cannot be disregarded. In addition, it is necessary to make the entire circuit small enough that it can process signals at a concentrated constant for high-speed processing, but the circuit scale increases and there is deterioration of high-frequency properties when the number of components is increased. In this regard, the circuit structure in FIG. 2(b) is simple, but ordinary dual-power sources and differential amplifier 15 generally have a low gain band width and therefore, distortion at high frequency increases. That is, the circuit in front that uses an ordinary operational amplifier as shown in FIG. 2 has a disadvantage in that although low-frequency properties, including direct current, are good, high-frequency properties and the properties of noise and distortion are poor.
Moreover, when single-power source-type differential amplifier 16 shown in FIG. 3 is used, broad-band, low-noise, low-distortion signals can be processed with an amplifier only, but because it is a single-power source-type amplifier, in-phase voltage signals 101 are necessary. When the dual-power source-type differential amplifier 17 shown in FIG. 3(a) with a low gain band width is added in front of single-power source-type differential amplifier 16 in order to process these in-phase voltage signals 101 by addition, the high-frequency properties of the converter as a whole deteriorate. Moreover, when the single-power source-type differential amplifier 16 shown in FIG. 3(b) is used with AC coupling, the in-phase signals 101 can be processed by addition using a bias circuit housed inside amplifier 16 and the properties of broad band, low noise and low distortion can be realized. However, the low frequency component is cut out by capacitor 18 before signal input and therefore, direct current signals and low-frequency signals cannot be processed. In particular, a single power source-type differential amplifier cannot be used for the circuit in front of signal input to an ADC of measurement apparatuses and the like.
Thus, mechanical parts such as ADCs are intended to realize broad band, low noise and low distortion, but there is no circuit in front of the ADC for processing signals that range from low-frequency signals including direct current signals to high-frequency signals and realizing low noise and low distortion and therefore, there is a technical problem in that the performance of the entire system deteriorates.
The present invention solves the problems by providing a broad-band, low-noise, low-distortion single end to differential signal converter, i.e., a low-noise, low-distortion single end to differential signal converter that processes signals ranging from low-frequency signals that include direct current signals to high-frequency signals.